FRICTION

ou might have seen a driver of a

car or a truck slowing down the

vehicle at a traffic signal. You, too,

slow down your bicycle whenever

needed by applying brakes. Have you

ever thought why a vehicle slows

down when brakes are applied? Not

only vehicles, any object, moving over

the surface of another object slows

down when no external force is

applied on it. Finally it stops. Have

you not seen a moving ball on the

ground stopping after some time?

Why do we slip when we step on a

banana peel (Fig. 9.1)? Why is it

difficult to walk on a smooth and

wet floor?

Fig. 9.1 : A boy falls down when he steps on a

banana peel

9.1 Force of Friction

Activity 9.1

Gently push a book on a table

[Fig. 9.2(a)]. You observe that it stops

after moving for some distance.

Repeat this activity pushing the

book from the opposite direction

[Fig. 9.2, (b)]. Does the book stop

this time, too? Can you think of an

explanation? Can we say that a force

must be acting on the book

opposing its motion? This force is

called the force of friction.

Fig. 9.2 (a), (b) : Friction opposes relative

motion between the

surfaces of the book and

the table

(a)

(b)

You will find the answers to such

questions in this chapter.

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You saw that if you apply the force

along the left, friction acts along the

right. If you apply the force along the

right, the friction acts along the left

direction. In both cases the force opposes

the motion of the book. The force of

friction always opposes the applied force.

In the above activity, the force of

friction acts between the surface of the

book and the surface of the table.

Is the friction the same for all the

surfaces? Does it depend on the

smoothness of the surfaces? Let us find

out.

9.2 Factors affecting Friction

Activity 9.2

Tie a string around a brick. Pull the

brick by a spring balance (Fig. 9.3).

You need to apply some force. Note

down the reading on the spring

balance when the brick just

begins to move. It gives you a

measure of the force of friction

between the surface of the brick

and the floor.

for this difference? Repeat this

activity by wrapping a piece of jute

bag around the brick. What do you

observe?

Fig. 9.3 : A brick is being pulled by spring

balance

Now wrap a piece of polythene

around the brick and repeat the

activity. Do you observe any

difference in the readings of the

spring balance in the above two

cases? What might be the reason

Spring Balance

Spring balance is a device used for

measuring the force acting on an

object. It consists of a coiled spring

which gets stretched when a force is

applied to it. Stretching of the spring

is measured by a pointer moving on

a graduated scale. The reading on the

scale gives the magnitude of the force.

Activity 9.3

Make an inclined plane on a smooth

floor, or on a table. You may use a

wooden board supported by bricks,

or books. [Fig. 9.4 (a)]. Put a mark

with a pen at any point A on the

inclined plane. Now let a pencil cell

move down from this point. How far

does it move on the table before

coming to rest? Note down the

distance. Now spread a piece of cloth

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Fig. 9.5 : Surface irregularities

over the table. Make sure that there

are no wrinkles in the cloth. Try the

activity again [Fig. 9.4 (b)].

Does the distance covered depend on

the nature of the surface on which the

cell moves?

Could the smoothness of the surface

of the cell also affect the distance

travelled by it?

(a)

(b)

Repeat this activity by spreading

a thin layer of sand over the

table. Maintain the same slope

throughout the activity.

In which case is the distance covered

the minimum? Why is the distance

covered by the pencil cell different every

time. Try to reason why? Discuss the

result.

Fig. 9.4 : The pencil cell covers different

distances on different surfaces

I shall try the activity

by wrapping a piece of

sandpaper around

the cell.

Friction is caused by the

irregularities on the two surfaces in

contact. Even those surfaces which

appear very smooth have a large number

of minute irregularities on them

(Fig. 9.5). Irregularities on the two

surfaces lock into one another. When

we attempt to move any surface, we have

to apply a force to overcome interlocking.

On rough surfaces, there are a larger

number of irregularities. So the force of

friction is greater if a rough surface

is involved.

Mark A

Surface of book

Surface of table

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We see that the friction is caused by

the interlocking of irregularities in the

two surfaces. It is obvious that the force

of friction will increase if the two

surfaces are pressed harder. You can

experience it by dragging a mat when

nobody is sitting on it, and when a

preson is sitting on it.

friction and you find it somewhat easier

to move the box already in motion than

to get it started.

9.3 Friction : A Necessary

Evil

Recall now some of your experiences. Is

it easier to hold a kulhar

(earthen pot)

or a glass tumbler? Suppose the outer

surface of the tumbler is greasy, or has

a thin layer of cooking oil on it; would it

become easier or more difficult to hold

it? Just think : would it be possible to

hold the glass at all if there is no

friction?

Recall also how difficult it is to move

on a wet muddy track, or wet marble

floor. Can you imagine being able to walk

at all if there were no friction?

You could not write with pen or

pencil if there were no friction. When

your teacher is writing with chalk on

the blackboard, its rough surface rubs

off some chalk particles which stick to

Recall your experience when last time

you moved a heavy box from one place

to another (Fig. 9.6). If you have no such

experience, get that experience now.

What is easier — to move the box from

rest, or to move it when it is already in

motion?

The force required to overcome

friction at the instant an object starts

moving from rest is a measure of static

friction. On the other hand, the force

required to keep the object moving with

the same speed is a measure of sliding

friction.

When the box starts sliding, the

contact points on its surface, do not get

enough time to lock into the contact

points on the floor. So, the sliding

friction is slightly smaller than the static

Fig. 9.7 : A nail is fixed in the wall due to

friction

Fig. 9.6 : You have to push on the box to keep it

moving

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the black board. Could it happen if there

were no friction between the chalk and

the board?

If an object started moving, it would

never stop if there were no friction. Had

there been no friction between the tyres

of the automobiles and the road, they

could not be started or stopped or

turned to change the direction of

motion. You could not fix a nail on the

wall (Fig. 9.7) or tie a knot. Without

friction no building could be constructed.

Fig. 9.8 : Soles of shoes wear out due

to friction

Fig. 9.10 : Stricking a matchstick produces fire

by friction

produces heat. In fact, when a machine

is operated, heat generated causes much

wastage of energy. We shall discuss the

ways of minimising friction in the

following section.

Fig. 9.9 : Rubbing of your palms makes you

feel warm

On the other hand, friction is an evil,

too. It wears out the materials whether

they are screws, ball bearings or soles

of shoes (Fig. 9.8). You must have seen

worn-out steps of foot over-bridges at

railway stations.

Friction can also produce heat.

Vigorously rub your palms together for

a few minutes (Fig. 9.9). How do you

feel? When you strike a matchstick

against the rough surface, it catches fire

(Fig. 9.10).

You might have observed that the jar

of a mixer becomes hot when it is run

for a few minutes. You can cite various

other examples in which friction

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9.4 Increasing and Reducing

Friction

As you have seen in the previous section,

friction is desirable in some situations.

Have you ever thought why the sole

of your shoe is grooved [Fig. 9.11 (a)]? It

is done to provide the shoes better grip

on the floor, so that you can move safely.

Similarly, the treaded tyres of cars,trucks

and bulldozers provide better grip with

the ground.

In some situations, however, friction

is undesirable and we would want to

minimise it.

Why do you sprinkle fine powder on

the carrom board (Fig. 9.12)? You might

have noticed that when a few drops of

oil are poured on the hinges of a door,

the door moves smoothly. A bicycle and

a motor mechanic uses grease between

the moving parts of these machines. In

all the above cases, we want to reduce

Fig. 9.12 : Powder is sprinkled on the carrom

board to reduce friction

friction in order to increase efficiency.

When oil, grease or graphite is applied

between the moving part of a machine,

a thin layer is formed there and moving

surfaces do not directly rub against each

other (Fig. 9.13). Interlocking of

irregularities is avoided to a great extent.

Movement becomes smooth. The

substances which reduce friction are

called lubricants. In some machines, it

may not be advisable to use oil as

lubricant. An air cushion between the

moving parts is used to reduce friction.

We deliberately increase friction by

using brake pads in the brake system

of bicycles and automobiles. When you

are riding a bicycle, the brake pads do

not touch the wheels. But when you

press the brake lever, these pads arrest

the motion of the rim due to friction.

The wheel stops moving. Y

ou might have

seen that kabaddi players rub their

hands with soil for a better grip of their

opponents. Gymnasts apply some coarse

substance on their hands to increase

friction for better grip.

(a) (b)

Fig. 9.11 : (a) Soles of shoes and (b) tyres are

treaded to increase friction

Fig. 9.13 : Action of lubricant

Surface 1

Surface 2

Lubricant

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Activity 9.4

Take a few pencils which are

cylindrical in shape. Place them

parallel to each other on a table.

Place a thick book over it

(Fig. 9.15). Now push the book. You

observe the pencils rolling as the

book moves. Do you feel it easier to

move the book in this way than to

slide it? Do you think that

resistance to the motion of the book

has been reduced? Have you seen

heavy machinary being moved by

placing logs under it?

9.5 Wheels Reduce Friction

You must have seen attaches and other

pieces of luggage fitted with rollers. Even

a child can pull such pieces of luggage

(Fig. 9.14). Why is it so? Let us

find out.

Can we reduce friction to

zero by polishing surfaces

or using large amount of

lubricants?

Friction can never be

entirely eleminated. No

surface is perfectly smooth.

Some irregularities are

always there.

Fig. 9.15 : Motion of the book on rollers

When one body rolls over the surface

of another body, the resistance to its

motion is called rolling friction. Rolling

reduces friction. It is always easier to

roll than to slide a body over another.

That is the reason it is convenient to

pull luggage fitted with rollers. Can you

now understand why the wheel is said

to be one of the greatest inventions of

mankind?

Since the rolling friction is smaller

than the sliding friction, sliding is

replaced in most machines by rolling

by the use of ball bearings. Common

examples are the use of ball bearings

between hubs and the axles of ceiling

fans and bicycles (Fig. 9.16).

Fig. 9.14 : Rolling reduces friction

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9.6 Fluid Friction

You know that air is very light and thin.

Yet it exerts frictional force on objects

moving through it. Similarly, water and

other liquids exert force of friction when

objects move through them. In science,

the common name of gases and liquids

is fluids

. So we can say that fluids exert

force of friction on objects in motion

through them.

The frictional force exerted by fluids

is also called drag.

The frictional force on an object in a

fluid depends on its speed with respect

to the fluid. The frictional force also

depends on the shape of the object and

the nature of the fluid.

It is obvious that when objects

move through fluids, they have to

overcome friction acting on them. In

this process they lose energy. Efforts

are, therefore, made to minimise

friction. So, objects are given special

shapes. Where do you think scientists

get hints for these special shapes?

From nature, of course. Birds and

fishes have to move about in fluids

all the time. Their bodies must have

evolved to shapes which would make

them lose less energy in overcoming

friction. Look carefully at the shape

of an aeroplane (Fig. 9.17). Do you

find any similarity in its shape and

that of a bird? In fact, all vehicles are

designed to have shapes which reduce

fluid friction.

Fig. 9.17 : Similarity in shapes of an aeroplane and a bird

Fig. 9.16 : Ball bearings reduce friction

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KEYWORDS KEYWORDS

KEYWORDS

BALL BEARING

DRAG

FLUID FRICTION

FRICTION

INTERLOCKING

LUBRICANTS

ROLLING FRICTION

SLIDING FRICTION

STATIC FRICTION

WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT

WHAT YOU HAVE LEARNT

Ü Friction opposes the relative motion between

two surfaces in contact. It acts on both the

surfaces.

Ü Friction depends on the nature of surfaces in

contact.

Ü For a given pair of surfaces friction depends

upon the state of smoothness of those

surfaces.

Ü Friction depends on how hard the two surfaces

press together.

Ü Static friction comes into play when we try to

move an object at rest.

Ü Sliding friction comes into play when an

object is sliding over another.

Ü Sliding friction is smaller than static friction.

Ü Friction is important for many of our activities.

Ü Friction can be increased by making a surface

rough.

Ü The sole of the shoes and the tyres of the

vehicle are treaded to increase friction.

Ü Friction is sometimes undesirable.

Ü Friction can be reduced by using lubricants.

Ü When one body rolls over another body, rolling

friction comes into play. Rolling friction is

smaller than sliding friction.

Ü In many machines, friction is reduced by

using ball bearings.

Ü Fluid friction can be minimised by giving

suitable shapes to bodies moving in fluids.

A riddle for you

In some situations, I oppose the motion

in other situations, I facilitate the motion

but, I always oppose

the relative motion

between two moving surfaces.

Put some lubricant, and

I become small there.

Make the moving surfaces rough

I make the movement tough.

I may be—

static, sliding or rolling

but whenever two surfaces are in motion

I am always there,

Tell me who I am!

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Exercises

1. Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact

with each other.

(b) Friction depends on the _____________ of surfaces.

(d) Sprinkling of powder on the carrom board ________ friction.

(e) Sliding friction is ___________ than the static friction.

2. Four children were asked to arrange forces due to rolling, static and sliding

frictions in a decreasing order

. Their arrangements are given below.

Choose the correct arrangement.

(a) rolling, static, sliding

(b) rolling, sliding, static

(d) sliding, static, rolling

3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper

and towel spread on the floor. The force of friction acting on the car on

different surfaces in increasing order will be

(a) wet marble floor, dry marble floor, newspaper and towel.

(b) newspaper, towel, dry marble floor, wet marble floor.

(d) wet marble floor, dry marble floor, towel, newspaper

4. Suppose your writing desk is tilted a little. A book kept on it starts sliding

down. Show the direction of frictional force acting on it.

5. You spill a bucket of soapy water on a marble floor accidently. Would it

make it easier or more difficult for you to walk on the floor? Why?

6. Explain why sportsmen use shoes with spikes.

7. Iqbal has to push a lighter box and Seema has to push a similar heavier

box on the same floor. Who will have to apply a larger force and why?

8. Explain why sliding friction is less than static friction.

9. Give examples to show that friction is both a friend and a foe.

10. Explain why objects moving in fluids must have special shapes.

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Extended Learning — Activities and Projects

1. What role does friction play in the sport of your choice? Collect

some pictures of that sport in action where friction is either

supporting it or opposing it. Display these pictures with proper

captions on the bulletin board of your classroom.

2. Imagine that friction suddenly vanishes. How would life be affected.

List ten such situations.

3. Visit a shop which sells sports shoes. Observe the soles of shoes

meant for various sports. Describe your observations.

4. A toy to play with:

Take an empty match box. Take out its tray. Cut a used refill of a ball

pen of the same width as the tray as shown in the figure below. Fix

the refill with two pins on the top of the tray as shown in Fig. 9.18.

Make two holes on the opposite sides of the tray. Make sure that the

holes are large enough to allow a thread to pass through them easily.

Take a thread about a metre long and pass it through the holes as

shown. Fix beads at the two ends of the thread so that it does not

come out. Insert the tray in the outer cover of the matchbox.

Suspend the match box by the thread. Leave the thread loose.

The match box will start falling down due to gravity. Tighten the

thread now and observe what happens.

Explain your observation. Can you relate it to friction?

Fig. 9.18

You can read more on the related topic on the following websites :

l http://www.school-for-champions.com/science/friction.htm

l http//hyperphysics.phy-astr.gsu.edu/hbase/firct2.html

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